Comment on: Doubly hybrid density functional xDH-PBE0 from a parameter-free global hybrid model PBE0 (J. Chem. Phys. 136, 174103 (2012))

Abstract: We have compared the performance of Grimme style DH/DSD and Zhang-Xu-Goddard type xDH/xDSD forms for double hybrids. In the DH and DSD forms, KS orbitals with elevated HF exchange and damped DFT correlation are used, while in the xDH and xDSD forms, the KS orbitals are obtained from a conventional hybrid functional with undamped DFT correlation. Generally, the difference in performance between DSD and xDSD functionals is small, slightly favoring xDSD. Augmentation of the xDH form with either same-spin MP2 correlation or a dispersion correction markedly improves performance. Best xDSD results appear to be obtained for orbitals obtained with `exact exchange' fractions in the 50-70% range. The orbitals for xDSD appear to be fairly transferable between different correlation functionals.